(1) Field of the Invention
The present invention relates to a polyester vessel for a drink and a process for the preparation thereof. More particularly, the present invention relates to a draw-blow-formed vessel which has a reduced shrinkage with time and is excellent in mechanical strength characteristics such as the pressure-resistant strength, and a process for the preparation thereof.
(2) Description of the Related Art
A biaxially drawn blow-formed vessel of a thermoplastic resin such as polyethylene terephthalate (PET) has excellent transparency and surface gloss and has properties required for a bottle, such as a good impact resistance, a high rigidity and a good gas-barrier property. Accordingly, this vessel is widely used for bottling various drinks.
In the production of a draw-blow-formed PET vessel, in general, a PET resin is injection-molded into a substantially amorphous bottomed preform, the bottomed preform is preliminarily heated at a drawing temperature, the preliminarily heated preform is stretched and drawn in the axial direction by a drawing rod while holding the preform in a split mold, and simultaneously, the preform is drawn in the circumferential direction by blowing a fluid into the preform. It is known that in order to prevent thermal shrinkage of the barrel of the vessel, the biaxially drawn vessel is thermally set according to need.
Preliminary heating of the preform is generally performed by supporting the preform by a mandrel or the like and irradiating the preform from the outer surface side with infrared rays. In this case, the temperature on the outer surface side of the perform is higher than the temperature on the inner surface side, and in order to avoid this phenomenon, it is necessary that uniform heating should be conducted for an extremely long time or a particular uniformalizing region should be disposed for eliminating the temperature difference between the outer surface side and inner surface side.
As the means for overcoming this disadvantage, there is known a method in which the preform is heated either from the inner surface side or from the outer surface side to uniformalize the temperature and shorten the heating time. For example, Japanese Examined Patent Publication No. 62-42852 teaches a method in which a layer of a heat-insulating material capable of generating heat under irradiation with far infrared rays is arranged on the periphery of a core shaft to be inserted into the interior of the perform and the preform is heated from the interior. Furthermore, Japanese Unexamined Patent Publication No. 62-77919 teaches that a radiation distribution suitable for heating a parison is obtained by forming a large-diameter portion at the end of a rod-shaped heater to be inserted into the interior of the parison. Moreover, Japanese Unexamined Patent Publication No. 61-163828 discloses a method in which a preform is heated from the outer side in a heating furnace and simultaneously, the preform is heated from the interior by a heat pipe inserted into the preform.
The above-mentioned conventional heating method comprising heating a preform the outer surface side and from the inner surface side is satisfactory in attaining the object of uniformalizing the temperature on the inner surface side and outer surface side and shortening the heating time, but the draw-blow-formed vessel actually prepared according to this method comes to have a large creep and is defective in that the pressure-resistant strength is considerably reduced, though the shrinkage with time is small.
A non-heat-set biaxially drawn polyester vessel according to the above-mentioned former conventional method has an excellent pressure-resistant strength, but the inner volume diminishes with the lapse of time and an inner volume necessary for filling a content cannot be ensured. Namely, this vessel is not satisfactory in the resistance to shrinkage with time.
A heat-set, biaxially drawn polyester vessel according to the latter conventional method is thermally dimensionstable and is excellent in the resistance to shrinkage with time, but the method is defective in that a heat-setting operation is necessary in addition to the blow-draw-forming operation, the mold occupancy time is long and another mold becomes necessary the heat setting, and therefore, the productivity is low and the manufacturing cost increases. For these reasons, a heat-set, biaxially drawn polyester vessel is used in the field where a content such as juice or oolong tea is hot-filled or heat-sterilized, but it is not used for carbonated drinks and the like.
Accordingly, in the field of vessels for carbonated refreshing drinks, development of a biaxially drawn polyester vessel having a pressure-resistant strength comparable or superior to that of the non-heat-set, biaxially drawn polyester vessel and having a shrinkage with time, which is controlled to a very low level without the heat-setting operation, is eagerly desired.